The present invention relates to an exhaust emission control device of an internal-combustion engine, and more specifically, to a technique for reducing the delivery of harmful substances from an internal-combustion engine by using secondary air supply means.
An exhaust emission control technique is known that utilizes reaction on a catalyst in order to reduce harmful substances (including smoke, NOx, etc. as well as unburned substances such as HC, CO, H2, etc.) in exhaust gas. Also known is a secondary air technique in which a catalyst is activated early by supplying secondary air to an exhaust port.
In some cases, however, harmful substances that are discharged before the activation of the catalyst amount to 90% of the total delivery in a cold mode, and arouse a critical problem.
Described in Jpn. Pat. Appln. KOKAI Publications Nos. 3-117611 and 4-183921, therefore, is a developed technique such that the exhaust pressure is raised in the cold state to activate the catalyst early. As is described in Jpn. Pat. Appln. KOKAI Publication No. 8-158897, for example, a technique is developed such that the catalyst is activated early by raising the exhaust pressure and supplying secondary air in the cold state.
According to an experiment conducted by the applicant hereof, it was confirmed that reaction in an exhaust system can be accelerated to enhance the exhaust gas purifying capability and improve the exhaust emission control efficiency by using the secondary air technique in combination with any of the exhaust pressure raising techniques described above.
However, an investigation made afterward by the applicant thereof indicates that the exhaust emission control performance lowers if the secondary air is supplied and the exhaust pressure is raised in the cold state with the combustion air-fuel ratio of the internal-combustion engine kept at a normal combustion air-fuel ratio for the case where the exhaust pressure is not raised.
If the fuel injection is carried out in an intake stroke so that the combustion air-fuel ratio in the cold state is a rich air-fuel ratio, in the case of a cylinder-injection internal-combustion engine, moreover, exhaust gas that contains plenty of hydrocarbon (HC), an unburned fuel, is discharged into exhaust passages. Since HC reacts less easily than carbon monoxide (CO) does, in general, the exhaust emission control performance is not improved very much although the secondary air supply and exhaust pressure raising are carried out.
In order to accelerate reaction with the secondary air in the exhaust system satisfactorily, on the other hand, the exhaust pressure should be raised to about 700 mmHg (933 hPa). Since the discharge pressure of an air pump that is generally used for the purpose of secondary air supply is about 150 mmHg (200 hPa), however, the air pump is expected to be considerably improved in performance. If the air pump is large-sized, there is the problem of increase in cost as well as in driving power consumption.
In this case, an attempt may be made to increase the internal EGR by raising the exhaust pressure and changing the overlap opening of an exhaust valve without using any secondary air, as is described in Jpn. Pat. Appln. KOKAI Publications Nos. 5-231195 and 8-158897, for example. However, the exhaust pressure raising technique and the increase of the internal EGR alone cannot satisfactorily accelerate the reaction in the exhaust system. Thus, it is hard to improve the exhaust emission control efficiency and fully activate the catalyst early.
The object of the present invention is to provide an exhaust emission control device of an internal-combustion engine, in which secondary air can be securely fed into an exhaust system with use of a low-priced configuration despite a rise in exhaust pressure, and reaction in the exhaust system can be satisfactorily accelerated at the start of operation of the internal-combustion engine, so that the exhaust emission control efficiency can be improved.
In order to achieve the above object, an exhaust emission control device of an internal-combustion engine according to the present invention comprises exhaust flow control means for restraining an exhaust flow so as to enhance the effect of reduction of the delivery of harmful substances at the start of operation of the internal-combustion engine, secondary air supply means for supplying secondary air to an exhaust system of the internal-combustion engine at the start of operation of the internal-combustion engine, and air-fuel ratio control means for adjusting the combustion air-fuel ratio of the internal-combustion engine to a rich air-fuel ratio at the start of operation of the internal-combustion engine.
In the case where the secondary air is supplied and the exhaust flow is restrained (to raise the exhaust pressure) at the start of operation of the internal-combustion engine, therefore, reaction in the exhaust system (including a combustion chamber, exhaust port, exhaust manifold, exhaust pipes, etc.) can be accelerated to improve the exhaust emission control efficiency by setting the combustion air-fuel ratio (air-fuel ratio before the secondary air supply) within the range of rich air-fuel ratios.
In the exhaust emission control device of the internal-combustion engine of the present invention, moreover, the air-fuel ratio control means controls the combustion air-fuel ratio of the internal-combustion engine so that the combustion air-fuel ratio is not lower than a combustion limit air-fuel ratio and not higher than 13.
In the case where the secondary air is supplied and the exhaust flow is restrained (to raise the exhaust pressure) at the start of operation of the internal-combustion engine, therefore, the exhaust emission control performance can be optimized by setting the combustion air-fuel ratio (air-fuel ratio before the secondary air supply) within the range of relatively high rich air-fuel ratios not lower than the combustion limit air-fuel ratio and not higher than 13.
Referring to FIG. 5, there are shown relations between the combustion air-fuel ratio (combustion A/F) before the secondary air supply and the HC delivery for each exhaust pressure as the results of measurement in 10 seconds after the start of operation of the internal-combustion engine. In this drawing, a two-dot chain line, dashed line, broken line, and full line represent cases of exhaust pressures of 0 mmHg (0 hPa), 300 mmHg (400 hPa), 500 mmHg (667 hPa), and 700 mmHg (933 hPa), respectively. If the exhaust pressure rises in this manner, the HC delivery is reduced in general within the range of rich air-fuel ratios not higher than 13, in particular.
This phenomenon is supposed to occur for the following reason. The rise of the exhaust pressure heightens the exhaust gas density. As the air-fuel ratio is enriched, the quantity of unburned substances discharged from the combustion chamber increases correspondingly, the probability of reaction in the exhaust system (including the combustion chamber, exhaust port, exhaust manifold, and exhaust pipes) increases, and the exhaust gas flows back from the exhaust port into the combustion chamber. As the gas in the combustion chamber is stirred, oxidation of unburned HC and the like are accelerated.
In the exhaust emission control device of the internal-combustion engine of the present invention, moreover, the secondary air supply means supplies the secondary air so that the exhaust air-fuel ratio obtained after the secondary air supply is a lean air-fuel ratio.
Thus, the reaction in the exhaust system can be further accelerated to improve the exhaust emission control efficiency additionally by supplying the secondary air so that the combustion air-fuel ratio is a rich air-fuel ratio and the exhaust air-fuel ratio after the secondary air supply is a lean air-fuel ratio.
In the exhaust emission control device of the internal-combustion engine of the present invention, furthermore, the secondary air supply means supplies the secondary air so that the exhaust air-fuel ratio obtained after the secondary air supply ranges from 18 to 22.
Thus, the exhaust emission control performance can be further optimized by supplying the secondary air so that the combustion air-fuel ratio is a rich air-fuel ratio and the exhaust air-fuel ratio after the secondary air supply ranges from 18 to 22.
Referring to FIG. 6, there is shown the relation between the exhaust air-fuel ratio (exhaust A/F) after the secondary air supply to the exhaust system and the HC delivery under the exhaust pressure of 700 mmHg (933 hPa) as the result of an experiment in 50 seconds after the start of operation of the internal-combustion engine. If the exhaust pressure is raised in this manner, the HC delivery has its minimum when the exhaust air-fuel ratio is 20 or thereabout, and can be specially lowered when the exhaust air-fuel ratio ranges from 18 to 22.
In the exhaust emission control device of the internal-combustion engine of the present invention, furthermore, the internal-combustion engine is a multi-cylinder internal-combustion engine, and the secondary air supply means supplies the secondary air by stopping fuel supply to some of cylinders or carrying out lean-A/F operation.
If the secondary air is supplied by stopping the fuel supply to some of the cylinders or carrying out the lean-A/F operation, therefore, plenty of oxygen is discharged from some cylinders, while great quantities of unburned substances are discharged with a rich air-fuel ratio from the other cylinders. Thus, when the exhaust pressure is raised, the unburned substances and oxygen react satisfactorily in the exhaust system, so that the exhaust emission control efficiency can be improved with ease.
In the exhaust emission control device of the internal-combustion engine of the present invention, moreover, the internal-combustion engine is a multi-cylinder internal-combustion engine and comprises two exhaust passages provided independently for each of two cylinder groups into which cylinders of the multi-cylinder internal-combustion engine are divided and communicating channels connecting the two exhaust passages, and the secondary air supply means includes exhaust oxygen quantity increasing means for increasing the quantity of oxygen in exhaust gas discharged from one of the two cylinder groups, and supplies the secondary air as the exhaust control means restrains the exhaust flow so that the degree of restraint of the flow of the exhaust gas discharged from the one cylinder group is higher than the degree of restraint of the flow of the exhaust gas discharged from the other cylinder group when the quantity of oxygen in the exhaust gas discharged from the one cylinder group is increased by means of the exhaust oxygen quantity increasing means.
Thus, the exhaust oxygen quantity increasing means is used to increase the quantity of oxygen in the exhaust gas discharged from the one cylinder group (e.g., to perform lean-A/F operation), and the exhaust flow is restrained so that the degree of restraint of the flow of the exhaust gas discharged from the one cylinder group (degree of rise of the exhaust pressure) is higher than the degree of restraint of the flow of the exhaust gas discharged from the other cylinder group. As this is done, the exhaust pressure of the oxygen-rich exhaust gas discharged from the one cylinder group becomes higher than the exhaust pressure of the exhaust gas discharged from the other cylinder group. Based on the resulting pressure difference, the oxygen-rich exhaust gas is supplied as the secondary air from the exhaust passages of the one cylinder group to the exhaust passages of the other cylinder group through the communicating channels.
Thus, in the case where the exhaust flow is restrained to raise the exhaust pressure, the secondary air can be supplied securely to accelerate the reaction in the exhaust system without using any large-capacity secondary air pump, so that the exhaust emission control efficiency can be improved with use of a simple configuration without any increase in cost.
In the exhaust emission control device of the internal-combustion engine of the present invention, furthermore, the internal-combustion engine is a multi-cylinder internal-combustion engine and comprises two exhaust passages provided independently for each of two cylinder groups into which cylinders of the multi-cylinder internal-combustion engine are divided and communicating channels connecting the two exhaust passages, and the secondary air supply means includes exhaust oxygen quantity increasing means for increasing the quantity of oxygen in exhaust gas discharged from one of the two cylinder groups and an air pump attached to the communicating channels and capable of force-feeding the exhaust gas from the exhaust passages of the one cylinder group to the exhaust passages of the other cylinder group.
Thus, the exhaust flow is restrained (to raise the exhaust pressure) by means of the exhaust flow control means, the quantity of oxygen in exhaust gas discharged from the one cylinder group is increased by means of the exhaust oxygen quantity increasing means, and the air pump is actuated. As this is done, the oxygen-rich exhaust gas is supplied as the secondary air from the exhaust passages of the one cylinder group to the exhaust passages of the other cylinder group through the communicating channels despite the rise in the exhaust pressure attributable to the restraint of the exhaust flow.
Thus, in the case where the exhaust flow is restrained to raise the exhaust pressure, the secondary air can be supplied securely to accelerate the reaction in the exhaust system by means of a small-capacity air pump, not a large-capacity secondary air pump, so that the exhaust emission control efficiency can be improved with use of a simple configuration without any increase in cost.
In the exhaust emission control device of the internal-combustion engine of the present invention, furthermore, the exhaust oxygen quantity increasing means stops fuel supply to one of the two cylinder groups or performs lean-A/F operation.
Thus, the quantity of oxygen discharged from the on cylinder group can be easily increased to feed the secondary air securely to the exhaust system by stopping the fuel supply to the one cylinder group or performing the lean-A/F operation.
Further, the exhaust emission control device of the internal-combustion engine of the present invention is an exhaust emission control device of an internal-combustion engine, which has an injection valve for injecting a fuel directly into a combustion chamber, fuel injection control means for controlling fuel injection by means of the injection valve, and air-fuel ratio control means for controlling the air-fuel ratio, comprising exhaust flow control means for restraining an exhaust flow so as to enhance the effect of reduction of the delivery of harmful substances at the start of operation of the internal-combustion engine, and secondary air supply means for supplying secondary air to the exhaust system of the internal-combustion engine at the start of operation of the internal-combustion engine, the fuel injection control means and the air-fuel ratio control means being adapted to inject the fuel in a compression stroke of the internal-combustion engine and to adjust the combustion air-fuel ratio to the theoretical air-fuel ratio or a rich air-fuel ratio, respectively, when the secondary air is supplied to the exhaust system by means of the secondary air supply means.
A conventional cylinder-injection internal-combustion engine in which a fuel is injected directly into a combustion chamber is believed to be liable to plug smoldering, since the fuel is injected into a region near a spark plug in compression-stroke injection. The applicant hereof conducted an experiment and found that compression-stroke injection with the internal-combustion engine in a cold state, in particular, was able to ensure a satisfactory spray condition, lessen the plug smoldering, and generate plenty of CO. In feeding the secondary air into the exhaust passages, based on this fact, the fuel is injected in the compression stroke of the internal-combustion engine to generate CO so that the combustion air-fuel ratio is the theoretical air-fuel ratio or a rich air-fuel ratio.
Thus, in the case where the secondary air is supplied and the exhaust flow is restrained (to raise the exhaust pressure) at the start of operation of the internal-combustion engine, the exhaust gas can be made to contain plenty of CO. Since CO reacts more easily than HC does, the reaction in the exhaust system can be satisfactorily accelerated to improve the exhaust emission control efficiency.